Many transplantable T-cell lymphomas contain amplified copies of endogenous mouse mammary tumor virus (MMTV) DNA. We suspect that this is a novel process unlike normal retrovirus infection because of the following. (i) There is size conservation of novel virus-cell junction fragments in these tumors, indicating site specificity of the amplified proviruses. (ii) Abelson-transformed clones can acquire amplified MMTV proviruses within a few in vivo passages. This pattern of acquired proviruses is then stable for long periods. (iii) No mature MMTV proteins, or viral particles, are detectable in C57BL (B6) lymphomas, suggesting that particle infection is not required for amplification. (iv) Rearrangements of endogenous proviruses are detected in the gag-pol region. Such rearrangements have not been detected in MMTV-infected mammary tumors. The mechanism suggested for the amplification process are gene amplification (generation of tandem head-to-tail repeats), reintegration through RNA intermediates, reintegration through DNA intermediates (transposition), and gene conversion. I propose to distinguish between these models by a series of experiments which include restriction mapping and subcloning of MMTV proviruses and their flanking sequences from a cloned library of B6 DNA. An in vitro system for studying MMTV amplification also will be developed. A secondary objective of this proposal is to study the function of amplified MMTV in lymphomas. This will be accomplished by mapping MMTV proviral sites by somatic cell hybridization. In addition transcription of MMTV proviruses and their flanking sequences will be monitored. Study of the amplification of endogenous MMTV DNA is important for achieving the long-term objective of understanding the transmission and regulation of endogenous viruses.